Human cytomegalovirus (hCMV) is a widely distributed pathogen that may cause severe pathology in immunosuppressed adults and upon infection of the fetus and has been implicated in chronic diseases such as atherosclerosis. hCMV infects multiple cell types including fibroblasts, endothelial, epithelial and hematopoietic cells [1]. In vitro propagated attenuated strains of hCMV, which are being developed as candidate vaccines, have lost the tropism for endothelial cells, while retaining the capacity to infect fibroblasts [2]. Two viral glycoprotein complexes are believed to control the cellular tropism of hCMV. A complex of glycoproteins such as gH, gL and gO appears to be required for infection of fibroblasts, while a complex of gH, gL and proteins encoded by the UL131-UL128 genes is implicated in infection of endothelial cells, epithelial cells and dendritic cells [2-8].
Hyperimmune globulins are already commercialized for the prophylaxis of hCMV disease associated with transplantation and recent evidence indicates that they have therapeutic effect in pregnant women[9]. This therapeutic approach is limited by the low amount of neutralizing antibody that can be transferred and for this reason the availability of human antibodies (such as human monoclonal antibodies) with high neutralizing capacity would be highly desirable. Although some antibodies to gH, gB and UL128 and UL130 gene products have demonstrated in vitro neutralizing activities [7, 10, 11] and an antibody to gH was evaluated in clinical trials (that were discontinued due to lack of therapeutic effects), the neutralizing potency of the antibodies isolated so far is modest. Neutralization by these antibodies was observed at antibody concentrations ranging from 0.5 to 20 μg/ml. Further, the current methods typically measure the neutralizing potency of anti-hCMV antibodies using fibroblasts as target cells. However, hCMV is also known to cause pathology by infecting other cell types such as endothelial, epithelial cells and leukocytes. Known antibodies to UL128 and UL130 show very low potency in neutralizing infection of endothelial cells [7] and there do not appear to be any monoclonal antibodies available that would be capable of neutralizing infection of non-fibroblast target cells with high potency.
There is therefore a need for antibodies that neutralize hCMV infection, particularly hCMV infection of non-fibroblast target cells, with high potency, as well as the elucidation of the target(s) to which such antibodies bind.